Hydrostatic transmission bypass latch

ABSTRACT

A hydrostatic transmission may be placed in a free-wheeling state by rotating a bypass actuator that lifts the motor from a motor running surface whereby hydraulic fluid flows into a transmission cavity. A bypass arm connected to the bypass actuator and to a bypass rod may be used to engage and disengage the bypass actuator via operation of the rod. Further, a bypass latch may be used to lock the bypass arm in an engaged position whereby the bypass actuator is engaged and the transmission is disengaged. The bypass latch arm is preferably linked to the brake pedal whereby actuation of the brake pedal disengages the bypass actuator through operation of the latching arm. The system may be designed to provide dynamic braking through transmission resistivity prior to application of a brake force to the axles or wheels by unlatching the bypass arm prior to actuating the brake arm.

FIELD OF THE INVENTION

The present invention relates generally to hydrostatic transmissions(“HST”) and transaxles including hydrostatic transmissions, and inparticular to a bypass system for such transmissions. Specific exemplaryembodiments discussed herein relate to riding lawn mowers and similarsmall tractors.

BACKGROUND OF THE INVENTION

The description of art in this section is not intended to constitute anadmission that any patent, publication or other information referred toherein is “prior art” with respect to this invention, unlessspecifically designated as such.

Riding lawn mowers and small tractors 10 of the type discussed hereinand depicted in FIGS. 1A and 1B generally use an engine having avertical output shaft 12 that is connected to the transaxle 14 via aconventional belt and pulley system 16. A standard HST for such atransaxle includes a hydraulic pump that is driven by the engine outputshaft 12, and a hydraulic motor, both of which are usually mounted on acenter section. Rotation of the pump by an input shaft creates an axialmotion for the pump pistons. The oil pressure created by this axialmotion is channeled via porting to the hydraulic motor, where it isreceived by the motor pistons. The axial motion of these pistons againsta thrust bearing causes the motor to rotate. The hydraulic motor in turnhas an output shaft that drives the vehicle axles through differentialgearing. For additional background, the reader is referred to U.S. Pat.No. 5,201,692, (“the '692 patent ”) issued to Johnson et al., andincorporated herein by reference in its entirety.

A problem with the typical HST arrangement is that a “neutral gear” doesnot exist, as it is merely a point where the hydraulic pressure in thepump goes to zero. However, at this point the oil remains in thetransmission, preventing the vehicle from being freely rolled. Thepresent invention is an improvement over prior art methods used to placea transaxle into neutral gear and enable movement of the tractor withoutthe motor running or, more generally, without the transmission beingengaged.

Prior art has solved the problem by diverting the oil through ahydraulic valve from the pressure side to the vacuum side of the HSTcenter section. The problem with such a design is that the hydraulicvalve allows for the movement of only a limited amount of oil due toinherent design limitations, such as the diameter of the hydraulic valvethrough which the oil is diverted. Machining such a valve also requiresprecise tolerances, thus increasing the manufacturing costs of the unit.

The '692 patent solves this neutral gear problem by providing amechanism whereby the motor block is mechanically lifted from itsrunning surface. This mechanism allows the oil to bypass thevacuum-pressure (hydraulic) circuit and to exit the case completely.This mechanism operates to enable the vehicle to free-wheel more easilythan is possible with prior art hydraulic valve methods.

FIG. 1C (which is similar to FIG. 2 of the '692 patent) is a sectionview through the transaxle 14. To activate the bypass feature disclosedin the '692 patent, a bypass arm 18 is manipulated by the user to rotatea bypass actuator 20. The bypass actuator 20 includes a rod 22 which isshaped at its base 24 so that rotation of the rod 22 forces a bypassplate 26 to press against the base of the motor 29, thereby breaking itsseal to the motor running surface (See FIGS. 2 and 5 of the '692patent). It will be apparent to those of skill in the art that a pin ormultiple pins, or other mechanical means may be used to lift the motorblock. These mechanical lifting mechanisms allow the oil to flow betweenthe motor and the transmission cavity.

A bypass rod 28 is connected to the bypass arm 18 to facilitatemanipulation of the bypass arm 18. FIG. 1D shows the bypass rod 28 in anunlocked position. The prior art means for activating the bypassmechanism includes pulling the bypass rod 28 through the tractor hitchplate 30 and securing it in place with a weldment (or cross pin) 32. Tosecure the bypass rod 28 in the hitch plate 30, the hitch plate 30 mustinclude a relatively complex stamp out, e.g., a keyhole 34. See FIG. 1Fwhich shows the bypass rod 28 extending through the keyhole 34 of thehitch plate 30. Perhaps more troublesome is the requirement to havetight tolerances for setting the cross pin 32 relative to the hitchplate 30 and the keyhole 34. This tolerance requirement is especiallyfrustrating because manufacturing tolerances involving the hitch plate30 and the vehicle frame are much looser.

While the present invention relates to hydrostatic transaxles andtransmissions generally, it will be better understood within thediscussion of exemplary embodiments directed toward riding lawnmowersand similar small tractors.

SUMMARY OF THE INVENTION

A primary object of the present invention is directed toward a bypasssystem, and in particular, a bypass latch, for a hydrostatic transaxleor transmission. A hydrostatic transaxle according to one embodiment ofthe present invention comprises a casing including a hydrostatictransmission located in the casing. The transmission comprises a motoron a motor running surface and a pump connected to the motor via ahydraulic circuit. A bypass actuator, in a preferred embodiment, isrotatably positioned to lift the motor from the motor running surfacewhen the actuator is rotated such that hydraulic fluid flows out of themotor (a part of the hydraulic circuit). Alternatively, other componentscan be shifted to allow fluid to flow out of the hydraulic circuit. Abypass arm is preferably positioned external to the casing (though neednot be) and affixed to the bypass actuator and rotatable therewith. Alatch arm that is rotatable about an axis parallel to an axis aboutwhich the bypass actuator rotates is adapted to releasably engage thebypass arm. The latch arm is coupled with a brake rod such thatoperation of the brake rod disengages the latch arm from the bypass arm,whereby the bypass actuator is disengaged and the transmission isthereby engaged.

Accordingly, an embodiment of the invention is directed toward a bypassassembly comprising a latch arm having a latching end for releasablyengaging the bypass arm and a brake end coupled with a brake rod. Areturning spring, preferably an extension spring, is connected to thelatch arm to apply a returning force to bias the latch arm toward anat-rest position. In a preferred embodiment the at-rest position is theposition where the brake end of the latch arm abuts, or is stoppedagainst, a brake arm. A second spring, preferably a compression spring,cooperating with the brake rod is used to apply a braking force to thebrake arm when the brake rod is actuated. In a preferred embodiment, thecompression spring transmits a force to the braking end of the latch armprior to transmitting a braking force to the wheels or axles of thetractor. A slidable member adapted to control the timing of the forcemay be positioned on the brake rod.

One advantage of the design is that it is integral with thetransmission. One method of integrating a bypass latch with ahydrostatic transmission according to the present invention comprisesplacing the hydrostatic transmission in a casing and connecting a bypassactuator to the transmission. This allows the transmission to roll morefreely when the bypass actuator is engaged than when the bypass actuatoris disengaged. A bypass arm positioned external to the casing isconnected to operate the bypass actuator. A latch arm adapted toreleasably engage the bypass arm is positioned external to the casing aswell. The method of integrating further includes linking the latchingarm to a brake rod such that operation of the brake rod causes thelatching arm to release the bypass arm, whereby the bypass actuator isdisengaged.

By integrating the design, the manufacturer could provide a means toactuate the bypass that is less expensive than the prior art. Thecurrent means of activation involves a bent wire form with a weldment(or cross pin). A straight wire form, rod, or stamping could be usedwith the proposed invention thereby reducing tolerance requirements andcosts.

Another advantage of the invention is that it can be deactivated by thepressing of the brake pedal. Many vehicles require the operator to pressthe brake pedal prior to starting the tractor. The present invention,which deactivates the bypass actuator when the brake pedal is applied,insures that the transmission is ready for operation. This is particularuseful when the user has perhaps forgotten that the bypass has been left“on,” i.e. the transmission is disengaged.

Other objects and advantages in accordance with the present inventionwill be apparent to those of skill in the art from the teachingsdisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the interest of enabling one of skill in the art to practice theinvention, exemplary embodiments are shown and described. For clarity,details apparent to those of skill in the art without undueexperimentation are generally omitted from the drawings and description.

FIG. 1A depicts a plan view of a typical lawn tractor.

FIG. 1B shows a lawn tractor similar to that of FIG. 1A with a cutout toshow a transaxle connected to an engine output via a belt system.

FIG. 1C is a section view through a prior art transaxle showing a bypassactuator.

FIG. 1D is an enlarged view of FIG. 1B showing a prior art design formanipulating and locking a bypass arm.

FIG. 1E shows the bypass arm of FIG. 1D secured in an engaged positionvia a cross pin on the bypass rod.

FIG. 1F shows the bypass rod of FIG. 1E extending through a keyholestamp out in the tractor hitch plate.

FIG. 2 shows a bypass latch according to one embodiment of the presentinvention.

FIG. 3 depicts an enlarged view the transaxle of FIG. 2, showing thebypass arm disengaged i.e., transmission engaged.

FIG. 4 is similar to FIG. 3 but with the bypass arm in the “on” orlatched position.

FIG. 5 shows a rear view of a vehicle employing the present inventionsuch that only a circular stamp out is needed in the hitch plate toallow the bypass rod to pass through.

FIG. 6 shows a cutout view of FIG. 5 to more clearly show the transaxle.

FIG. 7 shows a top view of a transmission similar to that show in FIG.2, but with the bypass rod extending in the opposite direction.

FIG. 8 is a side view of the transmission shown in FIG. 7 showing thebypass in the “off” position.

FIG. 9 is a top view of the transmission shown in FIG. 7 with the bypassrod is being pushed to activate the bypass actuator and the bypass armis starting to engage the latch arm.

FIG. 10 shows a top view of the transmission shown in FIG. 9 with thebypass arm latched in the “on” position, thereby disengaging thetransmission.

FIG. 11 shows a top view of the transmission of FIG. 7 with the bypassarm being unlatched.

FIG. 12 is a side view of the transmission shown in FIG. 11 but with thebypass fully disengaged and the brake arm activated.

FIG. 13 is a perspective view of the bypass latch system according tothe present invention, showing the bypass in the on position.

FIG. 14 is an isometric view of an alternative bypass latch using adifferent latch arm with a latching end having a tab generallyperpendicular to the axis of the arm.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention is discussed in relation to lawn tractors,however, other uses will be apparent from the teachings disclosedherein. The present invention will be better understood from thefollowing detailed description of exemplary embodiments with referenceto the attached drawings, wherein like reference numerals and charactersrefer to like parts, and by reference to the following claims.

FIG. 2 depicts a transaxle 14 with bypass latch 36 shown installed inthe tractor 10. The bypass arm is shown in the “off” position or thedisengaged position. Some vehicles may have the transaxle 14 (or moregenerally the transmission) rotated 180 degrees from what is shown inFIG. 2. If the transaxle is rotated, then the bypass rod 28 may extendforward, rather than rearward as shown. The bypass rod 28 may also berotated relative to the transaxle 14 to provide access to the bypassactuator from another direction without changing the orientation of thetransaxle 14. Thus, a design wherein the bypass rod 28 extends out theback of vehicle 10 is only one of many options for providing customeraccess. For example, in an embodiment wherein the bypass rod 28 extendsto one of the sides, the attachment point of the bypass arm 18 need onlybe rotated 90 degrees to provide the desired coupling between the bypassactuator 20 and the bypass rod 28. Other mechanisms to attain thedesired rotary motion will be apparent to those of skill in the art.

FIG. 3 depicts an enlarged view of FIG. 2, showing a hydrostatictransaxle according to one embodiment of the present invention. Thetransaxle 14 comprises a casing 38 including a hydrostatic transmissionlocated in the casing (the transmission is not shown in FIG. 3).Reference should be made to the '692 patent for additional detailregarding the general background of the invention. The transmissioncomprises a motor on a motor running surface and a pump connected to themotor via a hydraulic circuit. A bypass actuator 20 is rotatablypositioned to lift a transmission component when the actuator 20 isrotated such that hydraulic fluid flows out of the hydraulic circuit. Ina preferred embodiment the motor 29 is lifted from the motor runningsurface when the bypass actuator 20 is rotated. A bypass arm 18 ispreferably positioned external to the casing 38 and affixed to thebypass actuator 20 and rotatable therewith. A latch arm 40, that isrotatable about an axis 42 (See FIG. 13) which is parallel to an axis 44about which the bypass actuator 20 rotates, is adapted to releasablyengage the bypass arm 18. The latch arm 40 is coupled with a brake rod46 such that operation of the brake rod 46 disengages the latch arm 40from the bypass arm 18, whereby the bypass actuator 20 is disengaged andthe transmission is thereby engaged as the motor (or other transmissioncomponent) reseats itself.

Accordingly, an embodiment of the invention is directed toward a bypassassembly 48 such as shown in FIG. 13. The assembly 48 comprises a latcharm 40 rotatable about an axis 42 parallel to an axis 44 about which thebypass arm 18 rotates. The latch arm 40 comprises a latching end 50 forreleasably engaging the bypass arm 18 and a brake end 52 coupled with abrake rod 46. In a preferred embodiment, the brake end 52 is slidablycoupled with the brake end 46, but other means for movably coupling thetwo will suffice. A return spring 54, preferably an extension spring, isconnected to the latch arm 40 to apply a returning force to bias thelatch arm 40 toward an at-rest position (See FIGS. 3 and 7). In apreferred embodiment, the at-rest position is the position where thebrake end 52 of the latch arm 40 abuts, or is stopped against, a brakearm 56; intervening structure, such as spacers is acceptable. The brakeend 52 need not directly rest against the brake arm 56. A compressionspring 58 cooperating with the brake rod 46 is used to apply a brakingforce to the brake arm 56 when the brake rod 46 is actuated. In apreferred embodiment, the compression spring 58 transmits a force to thebraking end 52 of the latch arm 40 (via a bushing 60 positioned on thebrake rod 46) prior to the compression spring 58 transmitting a brakingforce, via the brake arm 56, to the wheels 59 or axles of the tractor10.

More generally, the bushing 60 may be a slidable member adapted to applythe brake force (from the brake rod 46) to the latch arm 40 before thebrake arm 56 is activated. Preferably the slidable member is positionedon the brake rod 46 between the compression spring 58 and the brake arm56. In a preferred embodiment, the bushing 60 and the brake arm 56 aredesigned such that an end of the bushing will pass through an opening 62in the brake arm 56, but an opposing end of the bushing 60 will not passthrough the opening 62.

In FIGS. 3 and 7, the latch arm 40 is positioned against the brake arm56 and the spacer 60 (bushing) as well, i.e. the latch arm 40 is notdirectly against the brake arm 56. The bypass rod 28 is positionedrearward in FIG. 3 and forward in FIG. 7. The vehicle will not rolleasily in this condition, where the transmission is engaged, withoutengine input because of back driving the gears, the pump, the pulleys,etc. To engage the bypass actuator 20, in the orientation shown in FIG.3, the user pulls the bypass rod 28.

FIG. 4 shows the same transaxle 14 as FIG. 3 but with the bypass 20latched in the “on” position. Since the bypass is engaged, the vehiclemay be rolled relatively easier even though there is no engine input.FIG. 5 shows a rear view of the vehicle 10 showing the hitch plate 30.The stamp out through which the bypass rod 28 extends is not visiblebehind the end of the bypass rod 28. FIG. 6 is a cutout of FIG. 5 tomore clearly show the transaxle 14. The mower deck 11 is shown roughlyaligned with the wheel axles. Note that the preferred stamp out in thepresent invention is circular. The present invention avoids the need fora keyhole stamp out and for a cross pin on the bypass rod 28 to securethe bypass actuator 20 in an engaged position.

FIG. 7 shows a top view of a transmission similar to the transaxle ofFIG. 3 (with the axles removed), but with the bypass rod 28 extending inthe opposite direction, i.e., it is rotated 180° in a plane normal tothe axis 44 of the actuator 20. Accordingly, the bypass rod 28 would bepushed in toward the case 38 of transmission to activate the bypassactuator 20. Thus a rider could reach the rod 28 from the tractor seat64 (See, e.g., FIG. 1A). While activation of the actuator 20 has beendescribed in relation to pushing and pulling the bypass rod 28 relativeto the casing 38, this is only for convenience. Other methods formanipulating the bypass arm 18 are within the spirit of the inventionand will be understood by those of skill in the art. Furthermore, thelatching assembly 48 may be readily adapted to operate with other chosenmethods of bypass-arm manipulation so as to couple operation of thebrake to the latch. The bypass arm 18 is in the disengaged or “off”position and the latch arm 40 is at an at-rest position against thebrake arm 56 and directly against the spacer 60.

FIG. 8 is a side view of the transmission shown in FIG. 7; the bypass isin the “off” position. The compression spring 58 is not compressed andthe brake arm 56 is not engaged.

FIG. 9 is a top view of the transmission shown in FIG. 7. The bypass rod28 is shown being pushed to activate the bypass actuator 20. The bypassarm 18 is starting to engage the latch arm 40. The extension spring 54is being stretched and exerting a force on the latch arm 40 brake end 52to bias the latch arm 40 toward the at-rest position. Spring 54 alsoforces the latching end 50 in the opposite direction as that of thebrake end 52 since the latch arm 40 rotates about axis 42 (through bolt66) which is positioned between the brake end 52 and the latching end50.

FIG. 10 shows the bypass arm 18 latched in the “on” position, therebydisengaging the transmission so the tractor rolls relatively morefreely. The extension spring 54 exerts a force to maintain the latchingend 50 of the latching arm 40 in engagement with the bypass arm 18. Whenthe brake rod 46 is being actuated, the force transmitted to thelatching arm 40 (via the compression spring 58) will overcome the forcefrom the extension spring 54 and rotate the latch arm 40 out ofengagement with the bypass arm 18. The springs in the hydraulic motoract to return the bypass actuator 20, and the bypass arm 18, to adisengaged position.

FIG. 11 shows the bypass arm 18 being unlatched. The brake rod 46 isactivated (via operator foot pedal typically) moving the compressionspring 58 in contact with the bushing 60. As the brake rod 46 andcompression spring 58 move further forward, the bushing 60 slidesthrough the brake arm 56 and makes contact with the latch arm 40 at itsbrake end 52. As the bushing 60 (or spacer) applies a force to the latcharm 40, the latch arm 40 starts to disengage from the bypass arm 18. Thebrake arm 56 has not yet rotated, i.e., the compression spring 58 hasnot yet applied a force (or, a sufficient force) to the brake arm 56when the latch arm 40 disengages the bypass arm 18. As the compressionspring 58 on the brake rod 46 is further compressed (or pulled furtherforward by the brake rod 46), the brake arm 56 begins to be activated.Activating the brake arm 56 applies a braking force to the wheels 59 (oraxles depending on the system). Thus, a system that utilizes dynamicbraking (via the hydrostatic transmission) prior to conventionalwheel/axle braking is achieved. By removing or otherwise adapting thebushing 60 or the latch arm 40 or both, the latch arm 40 can be made todisengage the bypass arm 18 contemporaneously with the onset of thebrake arm 56 activation. The latching assembly can also be made todisengage the bypass arm 18 after activation of the brake arm 56 or, aspreviously discussed, before activation of the brake arm 56.

FIG. 12 is a side view of the transmission shown in FIG. 11 but with thebypass arm (not shown) fully disengaged, the compression spring 58 fullycompressed and the brake arm 56 activated. By comparison, FIG. 8 is asimilar view prior to the brake arm 56 being activated.

FIG. 13 is a view of a bypass latch assembly 48 according to anembodiment of the present invention showing the bypass mechanism 48 inthe engaged position, so that rod 22 is rotated to force plate 26towards the cylinder block of motor 29, thus lifting motor 29 off themotor running surface 51 of center section 53. FIG. 14 is an isometricview of a bypass assembly 48 with an alternative bypass latch arm 40′having a different latching end 50′ from that shown in FIG. 13. Thelatching end 50′ has a tab 68 extending generally perpendicular to theaxis of the arm 40′ (i.e., the tab 68 extends generally parallel to axis42). The tab 68 need not extend perpendicular to the axis of the latcharm 40 but may be at a predetermined angle measured relative to the axisof the latch arm 40 or measured relative to an axis parallel to axis 42.The tab 68 allows the manufacturer to use looser manufacturingtolerances while achieving the desired latching capability. For example,the bypass arm 18 may be allowed out of plane movement, rather thanbeing rigidly held to a motion in a common plane with the latching arm40. Another alternative comprises adapting the latching end of thebypass arm 18 to latchingly engage the latching arm 40 even though botharms move out of plane.

While the invention has been particularly shown and described withreference to particular embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention. The scope of the claimed invention is intended to be definedby following claims as they would be understood by one of ordinary skillin the art with appropriate reference to the specification, includingthe drawings, as warranted.

What is claimed is:
 1. A hydrostatic transaxle comprising: a casing; a hydrostatic transmission in the casing, the transmission comprising a motor on a motor running surface and a pump connected to the motor via a hydraulic circuit; a bypass actuator positioned to lift the motor from the motor running surface such that hydraulic fluid flows out of the hydraulic circuit; a bypass arm external to the casing and affixed to the bypass actuator and rotable therewith; a latch arm rotatable about an axis parallel to an axis aligned with the bypass actuator, the latch arm releasably engaged with the bypass arm; and a brake rod coupled with the latch arm such that operation of the brake rod disengages the latch arm from the bypass arm whereby the bypass actuator is disengaged.
 2. The transaxle of claim 1, comprising a return spring connected to the latch arm and positioned to bias the latch arm toward an at-rest position.
 3. The transaxle of claim 1, comprising a spring cooperating with the brake rod to apply a force to the latch arm to disengage the latch arm from the bypass actuator arm.
 4. The transaxle of claim 3, comprising a brake arm rotatably connected to the casing and a slidable member slidably engaging the brake rod and positioned between the latch arm and the spring, wherein the member is adapted to apply the force to the latch arm before the brake arm is activated.
 5. The transaxle of claim 4, wherein the slidable member is a bushing riding on the brake rod, the spring is a compression spring through which the brake rod passes, and the brake arm is positioned between the bushing and the latch arm and defines an opening adapted to allow the bushing to pass through.
 6. A method of integrating a bypass latch with a hydrostatic transmission comprising: placing the hydrostatic transmission in a casing; connecting a bypass actuator to the transmission to allow the transmission to roll more freely when the bypass actuator is engaged than when the bypass actuator is disengaged; positioning a bypass arm external to the casing and connecting the bypass arm to the bypass actuator to operate the bypass actuator; positioning a latch arm external to the casing and adapting the latch arm to releasably engage the bypass arm; and coupling the latching arm to a brake rod such that operation of the brake rod causes the latching arm to release the bypass arm, whereby the bypass actuator is disengaged and the transmission is engaged.
 7. The method of claim 6, comprising spring biasing the latching arm toward an at-rest position.
 8. The method of claim 6, comprising positioning a slidable member on the brake rod and adapting the slidable member to cooperate with a brake arm such that operation of the brake rod causes the slidable member to transmit force to the latch arm to release the bypass arm before the brake arm is actuated, whereby the bypass actuator is disengaged before the brake arm transfers a braking force to a wheel.
 9. A bypass assembly adapted to couple a bypass arm to a brake rod, wherein the brake rod is operably connected to a brake arm, and the brake arm and the bypass arm are attached external to a casing housing a transmission, the bypass assembly comprising: a latch arm rotatably mounted external to the casing and having a latching end adapted to releasably engage the bypass arm whereby the bypass arm may be releasably latched, the latch arm also having a brake end movably connected to the brake rod; a return spring positioned to bias the brake end of the latch arm toward the brake arm; and a compression spring mounted on the brake rod and positioned to apply a force on the latch arm such that the latch arm will disengage from the bypass arm when the brake rod is operated.
 10. The bypass assembly of claim 9, comprising a bushing slidably mounted on the brake rod and adapted to transmit force from the compression spring to the latch arm prior to transmitting force from the compression spring to the brake arm such that the bypass arm is unlatch prior to actuating the brake arm.
 11. The bypass assembly of claim 9, wherein the latching end of the latch arm comprises a tab adapted to engage the bypass arm. 